The invention relates generally to offshore seismic prospecting and more particularly to marine seismic cables having multiple sensors, such as hydrophones, geophones, and accelerometers, disposed along the lengths of the cables.
Marine seismic cables instrumented with hydrophones and other sensors are used in prospecting for oil and gas trapped under the sea floor. Ocean Bottom Cables (OBCs) are seismic cables that are laid on the sea floor by a deployment vessel. Hydrophones and other sensors in the OBCs are used to detect reflections of periodically emitted seismic waves off geologic structures under the sea floor. The reflected seismic waves detected by hydrophones and particle motion sensors in the OBCs are used to produce a map of the subsea formations. After deployment, OBCs have to be retrieved by the vessel and stored.
In retrieving the seismic cables, cable-handling systems pull the cable aboard the survey vessel. During this retrieval process, the cable is subjected to severe tension that creates high bending loads in rigid cable sections, such as at sensor housings, as they pass over curved surfaces of the cable-handling equipment. The high bending loads can damage the rigid sections or shorten the useful life of the cable.
Some seismic cables use hydrophones to detect variations in acoustic pressure and motion sensors, such as geophones and accelerometers, to detect particle motion caused by reflected seismic waves. The motion sensors are particularly sensitive to vibrations in the cable that can be transmitted along its stress members, which, in OBCs, are often metal cables. High-modulus fiber ropes are sometimes used as stress members to provide better acoustic isolation than conventional wire ropes.